Digital compensation for nonlinearities in a polar transmitter

ABSTRACT

A polar transmitter includes a digital processor coupled to receive a complex modulated digital signal and a feedback signal produced from the complex modulated digital signal and that is operable to compare the complex modulated digital signal to the feedback signal to determine an error signal indicative of a difference between the complex modulated digital signal and the feedback signal. The digital processor is further operable to produce a correction signal from the error signal and to add the correction signal to the complex modulated digital signal to produce a corrected complex modulated digital signal.

BACKGROUND

1. Technical Field

The present invention relates to wireless communications and, moreparticularly, wideband wireless communication systems.

2. Related Art

Modern wireless RF transmitters for applications, such as cellular,personal, and satellite communications, employ digital modulationschemes such as frequency shift keying (FSK) and phase shift keying(PSK), and variants thereof, often in combination with code divisionmultiple access (CDMA) communication. Independent of the particularcommunications scheme employed, the RF transmitter output signal,s_(RF)(t), can be represented mathematically as

s _(RF)(t)=r(t)cos(2πf _(c) t+θ(t))  (1)

where f_(c) denotes the RF carrier frequency, and the signal componentsr(t) and θ(t) are referred to as the envelope and phase of s_(RF)(t),respectively.

Some of the above mentioned communication schemes have constantenvelope, i.e.,

r(t)=R,

and these are thus referred to as constant-envelope communicationsschemes. In these communications schemes, θ(t) constitutes all of theinformation bearing part of the transmitted signal. Other communicationsschemes have envelopes (amplitudes) that vary with time and these arethus referred to as variable-envelope communications schemes. In thesecommunications schemes, both r(t) and θ(t) constitute informationbearing parts of the transmitted signal.

The most widespread standard in cellular wireless communications iscurrently the Global System for Mobile Communications (GSM). The GSMstandard employs Gaussian Minimum Shift Keying (GMSK), which is aconstant-envelope binary modulation scheme allowing raw transmission ata maximum rate of 270.83 kilobits per second (kbps). Even higher datarates are achieved in the specification of the Enhanced Data rates forGSM Evolution (EDGE) cellular telephony standard by selectively applyinga 3π/8 offset, 8-level PSK (8-PSK) modulation scheme. With thisvariable-envelope communication scheme, the maximum bit rate is tripledcompared to GSM, while the chosen pulse shaping ensures that the RFcarrier bandwidth is the same as that of GSM, allowing for the reuse ofthe GSM signal bandwidths.

As mentioned above, the 8-PSK modulation scheme of EDGE is an example ofa variable envelope communications scheme. A common transmitter used insuch variable-envelope modulation communications schemes is the polartransmitter. In a typical polar transmitter architecture, digitalbaseband data enters a digital processor that performs the necessarypulse shaping and modulation to some intermediate frequency (IF) carrierf_(IF) to generate digital amplitude-modulated and digitalphase-modulated signals. The digital amplitude-modulated signal is inputto a digital-to-analog converter (DAC), followed by a low pass filter(LPF), along an amplitude path, and the digital phase-modulated signalis input to another DAC, followed by another LPF, along a phase path.The output of the LPF on the amplitude path is an analog amplitudesignal, while the output of the LPF on the phase path is an analog phasesignal. The analog phase signal is input to a phase-locked loop (PLL) toenable the phase of the RF output signal to track the phase of theanalog phase signal. The RF output signal is modulated in a non-linearpower amplifier (PA) by the analog amplitude signal. Thus, in polartransmitter architectures, the phase component of the RF signal isamplified through the non-linear PA while the amplitude modulation isperformed at the output of the PA.

In practice, the power spectrum emitted from an EDGE polar transmitterwill not be ideal due to various imperfections in the RF transmittercircuitry. Thus, quality measures of the transmitter performance havebeen established as part of the EDGE standard and minimum requirementshave been set. One quality measure that relates to the RF signal powerspectrum is the so-called spectral mask. This mask represents themaximum allowable levels of the power spectrum as a function offrequency offset from the RF carrier in order for a given transmitter toqualify for EDGE certification. In other words, the spectral maskrequirements limit the amount of transmitter signal leakage into otherusers' signal spectrum. For example, at a frequency offset of 400 kHz(0.4 MHz), the maximum allowable emission level is −54 dB relative tothe carrier (dBc). Another RF transmitter quality measure of the EDGEstandard is the modulation accuracy, which relates the RF transmittermodulation performance to an ideal reference signal. Modulation accuracyis related to the so-called error vector magnitude (EVM), which is themagnitude of the difference between the actual transmitter output andthe ideal reference signal. The error vector is, in general, a complexquantity and hence can be viewed as a vector in the complex plane.Modulation accuracy is stated in root-mean-square (RMS), 95thpercentile, and peak values of the EVM and is specified as a percentage.For a given transmitter to qualify for EDGE certification, the RMS EVMmust be less than 9%, the 95th percentile of EVM values must be lessthan 15%, and the peak EVM value must be less than 30%.

One component of the RF circuitry that significantly affects theperformance of the transmitter is the power amplifier. There are threemain sources of nonlinearities in most power amplifiers that contributeto the degradation of both the spectral mask and the EVM. The firstsource is known as LO feed-through (LOFT). Within a polar transmitter,the RF phase-modulated signal is typically generated by up-convertingthe IF phase-modulated signal to the desired RF signal using a localoscillator generator (LO). As such, the RF phase-modulated signal iscommonly referred to as the LO signal. Ideally, the output of the poweramplifier includes only the product of the LO signal and theamplitude-modulated signal. However, due to imperfections in the poweramplifier, a portion of the LO signal may also appear at the poweramplifier output. This leakage of the LO signal affects the performanceof the transmitter by increasing both the spectral mask and the EVM.

The other sources of nonlinearities in the power amplifier are AM-AMdistortion and AM-PM distortion. As the amplitude of the output signalvaries, distortion is added to both the amplitude-modulated (AM) signaland the phase-modulated (PM) signal. For example, since the amount of LOleakage changes with the amplitude level of the output signal, whenamplitude modulation is applied to the power amplifier, there is avariation in the carrier's phase due to the leakage that is a functionof the carrier's envelope (amplitude). This effect is known as AM-PMdistortion, and is critical when the power amplifier operates at highoutput power level.

Therefore, what is needed is a polar transmitter architecture capable ofcompensating for nonlinearities in the power amplifier.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus and methods of operationthat are further described in the following Brief Description of theDrawings, the Detailed Description of the Invention, and the claims.Other features and advantages of the present invention will becomeapparent from the following detailed description of the invention madewith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredwith the following drawings, in which:

FIG. 1 is a functional block diagram illustrating a communication systemthat includes a plurality of base stations or access points (APs), aplurality of wireless communication devices and a network hardwarecomponent;

FIG. 2 is a schematic block diagram illustrating a wirelesscommunication device as a host device and an associated radio;

FIG. 3 is a schematic block diagram of an exemplary polar RFtransmitter, in accordance with embodiments of the present invention;

FIG. 4 is a schematic block diagram of an exemplary phase-locked loop(PLL) for use in a polar RF transmitter, in accordance with embodimentsof the present invention;

FIG. 5 is a schematic block diagram of an exemplary digital processorfor use in a polar RF transmitter, in accordance with embodiments of thepresent invention;

FIG. 6 is a schematic block diagram of an exemplary RF transceiverproviding digital compensation for nonlinearities in the power amplifierof the transmitter, in accordance with embodiments of the presentinvention;

FIG. 7 is a graph illustrating exemplary phase distortion produced by apower amplifier in a polar transmitter;

FIG. 8 is a graph illustrating exemplary amplitude distortion producedby a power amplifier in a polar transmitter;

FIG. 9-11 illustrate exemplary waveforms produced during the measurementand compensation of power amplifier nonlinearities; and

FIG. 12 is a flow chart illustrating a method in accordance withembodiments of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram illustrating a communication system10 that includes a plurality of base stations or access points (APs)12-16, a plurality of wireless communication devices 18-32 and a networkhardware component 34. The wireless communication devices 18-32 may belaptop computers 18 and 26, personal digital assistants 20 and 30,personal computers 24 and 32 and/or cellular telephones 22 and 28. Thedetails of the wireless communication devices will be described ingreater detail below with reference to FIGS. 2-5.

The base stations or APs 12-16 are operably coupled to the networkhardware component 34 via local area network (LAN) connections 36, 38and 40. The network hardware component 34, which may be a router,switch, bridge, modem, system controller, etc., provides a wide areanetwork connection 42 for the communication system 10. Each of the basestations or access points 12-16 has an associated antenna or antennaarray to communicate with the wireless communication devices in itsarea. Typically, the wireless communication devices 18-32 register withthe particular base station or access points 12-16 to receive servicesfrom the communication system 10. For direct connections (i.e.,point-to-point communications), wireless communication devicescommunicate directly via an allocated channel.

Typically, base stations are used for cellular telephone systems andlike-type systems, while access points are used for in-home orin-building wireless networks. Regardless of the particular type ofcommunication system, each wireless communication device and each of thebase stations or access points includes a built-in radio and/or iscoupled to a radio. The radio includes a transceiver (transmitter andreceiver) for modulating/demodulating information (data or speech) bitsinto a format that comports with the type of communication system.

FIG. 2 is a schematic block diagram illustrating a wirelesscommunication device 18-32 as a host device and an associated radio 60.For cellular telephone hosts, the radio 60 is a built-in component. Forpersonal digital assistants hosts, laptop hosts, and/or personalcomputer hosts, the radio 60 may be built-in or an externally coupledcomponent.

As illustrated, the host wireless communication device 18-32 includes aprocessing module 50, a memory 52, a radio interface 54, an inputinterface 58 and an output interface 56. The processing module 50 andmemory 52 execute the corresponding instructions that are typically doneby the host device. For example, for a cellular telephone host device,the processing module 50 performs the corresponding communicationfunctions in accordance with a particular cellular telephone standard.

The radio interface 54 allows data to be received from and sent to theradio 60. For data received from the radio 60 (e.g., inbound data), theradio interface 54 provides the data to the processing module 50 forfurther processing and/or routing to the output interface 56. The outputinterface 56 provides connectivity to an output device, such as adisplay, monitor, speakers, etc., such that the received data may bedisplayed. The radio interface 54 also provides data from the processingmodule 50 to the radio 60. The processing module 50 may receive theoutbound data from an input device, such as a keyboard, keypad,microphone, etc., via the input interface 58 or generate the dataitself. For data received via the input interface 58, the processingmodule 50 may perform a corresponding host function on the data and/orroute it to the radio 60 via the radio interface 54.

Radio 60 includes a host interface 62, a memory 75, a local oscillationmodule 74, a receiver 100, a transmitter 150, a transmitter/receiver(TX/RX) switch module 73 and an antenna 86. The receiver 100 includes adigital receiver processing module 64, an analog-to-digital converter66, a filtering/gain module 68, a down-conversion module 70, a low noiseamplifier 72 and a receiver filter module 71, while the transmitter 150includes a digital transmitter processing module 76, a digital-to-analogconverter 78, a filtering/gain module 80, an IF mixing up-conversionmodule 82, a power amplifier 84 and a transmitter filter module 85. Theantenna 86 is shared by the transmitter 150 and receiver 100, asregulated by the TX/RX switch module 73. The antenna implementation willdepend on the particular standard to which the wireless communicationdevice is compliant.

The digital receiver processing module 64 and the digital transmitterprocessing module 76, in combination with operational instructionsstored in memory 75, execute digital receiver functions and digitaltransmitter functions, respectively. The digital receiver functionsinclude, but are not limited to, demodulation, constellation demapping,decoding, and/or descrambling. The digital transmitter functionsinclude, but are not limited to, scrambling, encoding, constellationmapping, and/or modulation.

The digital receiver and transmitter processing modules 64 and 76,respectively, may be implemented using a shared processing device,individual processing devices, or a plurality of processing devices.Such a processing device may be a microprocessor, micro-controller,digital signal processor, microcomputer, central processing unit, fieldprogrammable gate array, programmable logic device, state machine, logiccircuitry, analog circuitry, digital circuitry, and/or any device thatmanipulates signals (analog and/or digital) based on operationalinstructions.

Memory 75 may be a single memory device or a plurality of memorydevices. Such a memory device may be a read-only memory, random accessmemory, volatile memory, non-volatile memory, static memory, dynamicmemory, flash memory, and/or any device that stores digital information.Note that when the digital receiver processing module 64 and/or thedigital transmitter processing module 76 implements one or more of itsfunctions via a state machine, analog circuitry, digital circuitry,and/or logic circuitry, the memory storing the corresponding operationalinstructions is embedded with the circuitry comprising the statemachine, analog circuitry, digital circuitry, and/or logic circuitry.Memory 75 stores, and the digital receiver processing module 64 and/orthe digital transmitter processing module 76 executes, operationalinstructions corresponding to at least some of the functions illustratedherein.

In operation, the radio 60 receives outbound data 94 from the hostwireless communication device 18-32 via the host interface 62. The hostinterface 62 routes the outbound data 94 to the digital transmitterprocessing module 76, which processes the outbound data 94 in accordancewith a particular wireless communication standard (e.g., GSM, EDGE,WCDMA, Bluetooth EDR, etc.) to produce digital transmission formatteddata 96. The digital transmission formatted data 96 is a digitalbaseband signal or a digital low IF signal, where the low IF typicallywill be in the frequency range of 100 KHz to a few Megahertz.

The digital-to-analog converter 78 converts the digital transmissionformatted data 96 from the digital domain to the analog domain. Thefiltering/gain module 80 filters and/or adjusts the gain of the analogbaseband signal prior to providing it to the up-conversion module 82.The up-conversion module 82 directly converts the analog basebandsignal, or low IF signal, into an RF signal based on a transmitter localoscillation 83 provided by local oscillation module 74. The poweramplifier 84 amplifies the RF signal to produce an outbound RF signal98, which is filtered by the transmitter filter module 85. The antenna86 transmits the outbound RF signal 98 to a targeted device, such as abase station, an access point and/or another wireless communicationdevice.

The radio 60 also receives an inbound RF signal 88 via the antenna 86,which was transmitted by a base station, an access point, or anotherwireless communication device. The antenna 86 provides the inbound RFsignal 88 to the receiver filter module 71 via the TX/RX switch module73, where the RX filter module 71 bandpass filters the inbound RF signal88. The RX filter module 71 provides the filtered RF signal to low noiseamplifier 72, which amplifies the inbound RF signal 88 to produce anamplified inbound RF signal. The low noise amplifier 72 provides theamplified inbound RF signal to the down-conversion module 70, whichdirectly converts the amplified inbound RF signal into an inbound low IFsignal or baseband signal based on a receiver local oscillation signal81 provided by local oscillation module 74. The down-conversion module70 provides the inbound low IF signal or baseband signal to thefiltering/gain module 68. The filtering/gain module 68 filters and/orattenuates the inbound low IF signal or the inbound baseband signal toproduce a filtered inbound signal.

The analog-to-digital converter 66 converts the filtered inbound signalfrom the analog domain to the digital domain to produce digitalreception formatted data 90. The digital receiver processing module 64decodes, descrambles, demaps, and/or demodulates the digital receptionformatted data 90 to recapture inbound data 92 in accordance with theparticular wireless communication standard being implemented by radio60. The host interface 62 provides the recaptured inbound data 92 to thehost wireless communication device 18-32 via the radio interface 54.

As one of average skill in the art will appreciate, the wirelesscommunication device of FIG. 2 may be implemented using one or moreintegrated circuits. For example, the host device may be implemented ona first integrated circuit, while the digital receiver processing module64, the digital transmitter processing module 76 and memory 75 areimplemented on a second integrated circuit, and the remaining componentsof the radio 60, less the antenna 86, may be implemented on a thirdintegrated circuit. As an alternate example, the radio 60 may beimplemented on a single integrated circuit. As yet another example, theprocessing module 50 of host device 18-32 and the digital receiverprocessing module 64 and the digital transmitter processing module 76 ofradio 60 may be a common processing device implemented on a singleintegrated circuit. Further, memory 52 and memory 75 may be implementedon a single integrated circuit and/or on the same integrated circuit asthe common processing modules of processing module 50, the digitalreceiver processing module 64, and the digital transmitter processingmodule 76.

FIG. 3 is a schematic block diagram of an exemplary polar RF transmitterarchitecture capable of compensating for nonlinearities in the poweramplifier 84 in accordance with embodiments of the present invention.The polar RF transmitter 150 shown FIG. 3 is functionally equivalent toblocks 76, 78, 80, 82 and 84 of FIG. 2. One typical application of theRF polar transmitter of FIG. 3 is EDGE, though the concepts may readilybe applied to other types of communication networks. In FIG. 3, it isassumed that the necessary pulse shaping, modulation, and interpolationfiltering has already been performed to produce a complex modulateddigital signal 110 with an envelope (amplitude) component and a phasecomponent.

The polar transmitter of FIG. 3 further includes a radio digitalprocessor 105, high sample rate digital-to-analog converters (DACs) 130and 132, low pass filters (LPFs) 134 and 136, a translational loop(e.g., a PLL) 138 and a power amplifier (PA) 84. In an exemplaryembodiment, the functionality of the power amplifier 84 is provided by acombination of a low power on-chip power amplifier driver (PAD) and ahigh power off-chip power amplifier. The on-chip power amplifierperforms the modulation of the RF signal, while the off-chip poweramplifier amplifies the modulated RF signal to the appropriate outputpower level. However, due to imperfections in the PAD, nonlinearities,such as LO feed-through (LOFT), AM-AM distortion and AM-PM distortion,may arise, thereby affecting the performance of the transmitter byincreasing both the spectral mask and the EVM of the transmitter.

Therefore, in accordance with embodiments of the present invention, afeedback loop is provided through the receiver 100 from the output ofthe PA 84 to the radio digital processor 105 to measure thenonlinearities in the PA 84 in a measurement mode and enable the radiodigital processor to compensate for the measured nonlinearities in anoperating mode. During the measurement mode of the polar transmitter150, the radio digital processor 105 receives a feedback signal 120 fromthe feedback loop that contains an envelope (amplitude) component and aphase component for comparison with the amplitude and phase componentsof the input complex digital modulated signal 110. Based on the feedbacksignal 120, the radio digital processor 105 measures the LOFT, AM-AM andAM-PM distortion of the PA 84 and stores the measurements for subsequentuse in the operating mode. In the operating mode, the radio digitalprocessor 105 pre-distorts the envelope and phase components of thecomplex modulated digital signal 110 based on the stored measurements toeffectively cancel the LOFT and AM/AM&PM distortions produced by the PA84.

In an exemplary measurement mode operation of the polar transmitter 150,the envelope and phase components of the complex modulated digitalsignal 110 are input to the radio digital processor 105 for processing.During the measurement mode, the complex modulated digital signal 110 isa test signal designed to assist the radio digital processor 105 inmeasuring the nonlinearities of the PA 84. The radio digital processor105 is further coupled to receive envelope and phase components of afeedback signal 120 from the output of the PA 84 via the receiver 100.The radio digital processor 105 operates to compare the complexmodulated digital signal 110 to the feedback signal 120 to measure anerror signal indicative of a difference between the complex modulateddigital signal 110 and the feedback signal 120. For example, whenmeasuring LOFT, the error signal can be a measure of the DC offset orbias that is added to the envelope path of the transmitter by the PA 84.As another example, when measuring AM/AM&PM distortion, the error signalcan be a measure of the variation of the amplitude and phase of theoutput of the PA 84 as a function of the amplitude of the input complexmodulated digital signal 110. The radio digital processor 105 stores theerror signal measurements for subsequent use in an operating mode.

In an exemplary operating mode of the polar transmitter 150, theenvelope and phase components of the complex modulated digital signal110 are input to the radio digital processor 105 for processing. Basedon the stored error measurements, the radio digital processor 105 isable to produce a correction signal that when added to the complexmodulated digital signal 110 produces a corrected complex modulateddigital signal including a corrected envelope signal 115 and a correctedphase signal 125 that digitally compensates for nonlinearities in the PA84.

In either mode, the digital envelope signal 115 output from the radiodigital processor 105 is input to high sample rate DAC 130, followed byLPF 134 to produce an analog envelope signal 140. In one embodiment, thedigital phase signal 125 output from the radio digital processor 105 isinput to high sample rate DAC 132, followed by LPF 136 to filter out anydigital images to produce a phase-modulated analog signal that isprovided to the input of the translational loop 138. The translationalloop 138 operates to up-convert the phase-modulated analog signal froman intermediate frequency (IF) to a radio frequency (RF) to produce anRF phase signal (output carrier) 145. In another embodiment, asillustrated by the dotted line, and as further described below inconnection with FIG. 4, the digital phase signal 125 output from theradio digital processor 105 is input directly to the translational loop138. In this embodiment, the translational loop is a fractional-Nphase-locked loop (PLL) that operates to produce the RF phase signal(output carrier) 145 such that the phase of the RF phase signal 145phase tracks the phase of the digital phase signal 125. The RF outputcarrier 145 is modulated in the PA 84 by the analog envelope signal 140to produce a modulated RF signal 148. In operating mode, the modulatedRF signal 148 is transmitted over an antenna (not shown), while inmeasurement mode, the modulated RF signal 148 is fed back to the radiodigital processor 105 through the receiver 100 as feedback signal 120.

FIG. 4 is a schematic block diagram of an exemplary phase-locked loop(PLL) for use in a polar RF transmitter. The PLL shown in FIG. 4includes a phase frequency detector (PFD) 414, a charge pump (CP) 418, alow pass filter (LPF) 422, a voltage controlled oscillator (VCO) 426, amulti-modulus divider (MMD) 428 and a ΔΣ MMD controller 430. ΔΣ MMDcontroller 430 is coupled to receive the digital phase-modulated signal125, and is operable to generate divider control signals 432 to the MMD428 based upon the digital phase-modulated signal 125. The MMD 428 iscoupled to receive the divider control signals 432 from the ΔΣ MMDcontroller 430 and is operable to produce a feedback signal 434 based onthe divider control signals 432.

The PFD 414 is coupled to receive a precise reference signal 412 from acrystal oscillator 410 for comparing with the feedback signal 434 toproduce an error signal 416 indicative of a phase or frequencydifference between the reference signal 412 and the feedback signal 434.The CP 418 produces current pulses 420 based upon the error signal 416,and provides the current pulses to LPF 422. LPF 422 filters the currentpulses 420 to produce a control voltage 424 that controls theoscillation of the VCO 426. In response to the control voltage 424, VCO426 produces an oscillation that is output as the RF phase signal 145.In addition, the oscillation 145 produced by the VCO 426 is fed back tothe MMD 428, which divides the oscillation 145 by a divider ratio toproduce the feedback signal 434 that is provided to the PFD 414. Asdescribed above, MMD 428 sets the divider ratio based upon the dividercontrol signal 432 received from the ΔΣ MMD controller 430, and ΔΣ MMDcontroller 430 generates the divider control signal 432 based upon thedigital phase-modulated signal 125.

In a practical setting, the VCO 426 typically undergoes “calibration” aspart of operating the PLL. This calibration sets the approximateoperating point of the VCO and allows the VCO to function over a widerange of frequencies. The VCO is typically calibrated for every channelhop. This calibration process involves a sequence of switching in andout of capacitors that tune the operation frequency of the VCO.Typically, calibration of a PLL occurs in two stages. Initially, an openloop stage serves to place the output oscillation with an approximatedeviation of a desired frequency of oscillation. The open loop stage isthen followed by a closed loop stage that locks the oscillation to adesired frequency of oscillation.

In a properly designed PLL, the feedback loop properties of the PLLresults in the VCO output 145 “locking” to a frequency equal to theproduct of crystal oscillator reference frequency 412 and the dividerratio of the MMD 428. Thus, the closed loop tracking action causes theerror signal 416 to approach zero, and therefore, the phase of the RFoutput carrier 145 tracks the phase of the digital phase-modulatedsignal 125, as desired.

FIG. 5 is a schematic block diagram of an exemplary digital processor105 for use in a polar RF transmitter, in accordance with embodiments ofthe present invention. The digital processor 105 includes an envelopecomparator 160 for comparing the envelope components of the complexmodulated digital signal 110 and the feedback signal 120 and a phasecomparator 162 for comparing the phase components of the complexmodulated digital signal 110 and feedback signal 120 during measurementmode. The output of the envelope comparator 160 is an envelope errorsignal 164 indicative of a difference in amplitude between the envelopecomponents of the complex modulated digital signal 110 and feedbacksignal 120. The output of the phase comparator 162 is a phase errorsignal 166 indicative of a difference in phase between the phasecomponents of the complex modulated digital signal 110 and feedbacksignal 120.

The envelope error signal 164 is stored in envelope correction logic 168and the phase error signal 166 is stored in phase correction logic 170for subsequent use during operating mode. Thus, during operating mode,the envelope component of the complex modulated digital signal 110 isinput to the envelope correction logic 168 and the phase component ofthe complex modulated digital signal 110 is input to the phasecorrection logic 170. The envelope correction logic 168 operates toproduce an envelope correction signal 172 based on the stored envelopeerror signal 164 and the envelope component of the complex modulateddigital signal 110, and the phase correction logic 170 operates toproduce a phase correction signal 174 based on the stored phase errorsignal 166 and the phase component of the complex modulated digitalsignal 110. The envelope correction signal 172 is added to the envelopecomponent of the complex modulated digital signal 110 by adder 176 toproduce the corrected envelope signal 115. The phase correction signal174 is added to the phase component of the complex modulated digitalsignal 110 by adder 178 to produce the corrected phase signal 125. Thus,the output of the digital processor 105 is a pre-distorted signal(corrected envelope 115 and corrected phase 125) that compensates fornonlinearities in the power amplifier.

FIG. 6 is a schematic block diagram of an exemplary RF transceiverproviding digital compensation for nonlinearities in the power amplifierof the polar transmitter, in accordance with embodiments of the presentinvention. The transceiver shown in FIG. 6 includes the radio digitalprocessor 105, a source generator 200, a power amplifier driver (PAD)225 and a feedback path including various receiver circuitry 230, 66 and64. The source generator 200 is operable to generate a control signal202 that controls the radio digital processor 105. For example, to beginthe measurement process, the source generator 200 enables a measurementmode in the radio digital processor 105. Similarly, to begin normaloperation of the radio digital processor, the source generator enablesan operating mode in the radio digital processor 105. In addition, whilein measurement mode, the source generator 200 is operable to generate acomplex digital test signal 110 including envelope and phase componentsand to provide the complex digital test signal 110 to the radio digitalprocessor 105.

The receiver circuitry includes RX front end circuitry 230 (e.g., a lownoise amplifier, down-conversion module and various filtering/gainmodules), a complex analog-to-digital converter 66, channel selectfilters 236, feedback measurement averaging filter 238 and a coordinaterotation digital computer (CORDIC) module 240. The complexanalog-to-digital converter (ADC) 300 is connected to receive an analogcomplex signal from the RX front end circuitry 230. The analog complexsignal includes analog in-phase and quadrature phase signals. The analogin-phase signal is received at a first ADC 232 of the complex ADC 66 andthe quadrature phase signal is received at a second ADC 234 of thecomplex ADC 66. The first ADC 232 converts the analog in-phase signalfrom the analog domain to the digital domain to produce a digitalin-phase signal. The second ADC 234 converts the analog quadrature phasesignal from the analog domain to the digital domain to produce a digitalquadrature phase signal.

The digital in-phase and quadrature-phase signals are input to thechannel select filters 236 and feedback measurement averaging filters236 to filter the digital in-phase and quadrature-phase signals toproduce digital filtered in-phase and quadrature-phase signals. In oneembodiment, the feedback measurement averaging filters 238 are low passfilters. The in-phase and quadrature-phase digital filtered signals areinput to the CORDIC module 240, which serves as a vector de-rotator tode-rotate the I and Q vector digital data. For example, in oneembodiment, the CORDIC module 240 can de-rotate the complex input vectorback down to the real axis to produce a digitized feedback signal 120representing the angle (phase) and magnitude (envelope) of the complexinput vector.

In an exemplary operation of the RF transceiver of FIG. 6, the sourcegenerator 200 first enables the measurement mode in the radio digitalprocessor 105 and selects the appropriate bandwidth using the controlsignal 202. While in measurement mode, the radio digital processor 105is connected to receive the feedback signal 120 via the feedback loopand a complex digital test signal 110 from the source generator 200. Inan exemplary embodiment, the complex digital test signal 110 includes asequence of test signals designed to measure the nonlinearities in thePAD 225. The complex digital test signal 110 is output to the PAD 225from the radio digital processor to produce a test RF signal 148, whichis fed back to the radio digital processor 105 via the RX front end 230,complex DAC's 232 and 234, channel select filters 236, feedbackmeasurement averaging filters 238 and CORDIC module 240.

The radio digital processor 105 compares the envelope and phasecomponents of the complex digital test signal 110 to the envelope andphase components of the feedback signal 120 to measure an error signalindicative of a difference therebetween. For example, the radio digitalprocessor 105 can first measure the LOFT by maintaining a constantenvelope test signal 110 and measuring the DC offset between the testsignal 110 and the feedback signal 120. Once the LOFT has been measured,the radio digital processor 105 can then measure the AM/AM& PMdistortion by allowing the complex digital test signal 110 to sweep thedynamic range of the polar transmitter and measuring the envelope andphase distortion profiles that indicate the variation of the amplitudeand phase of the output of the PAD 225 as a function of the amplitude ofthe input complex digital test signal 110.

For example, an exemplary profile of the phase distortion produced bythe PAD as a function of the amplitude of the input test signal is shownin FIG. 7. Ideally, the phase response of the PAD to changes in theamplitude of the input test signal should be linear. However, as can beseen in FIG. 7, the phase response of the PAD varies significantly withamplitude. In addition, an exemplary profile of the amplitude distortionproduced by the PAD as a function of the amplitude of the input testsignal is shown in FIG. 8. Although the profile shown in FIG. 8 appearsto be linear, there are actually small amounts of non-visiblenonlinearity present in the profile. Thus, the gradient of the curveshown in FIG. 8 varies slightly.

Referring again to FIG. 6, after the nonlinearities in the PAD 225 havebeen measured and stored, the source generator 200 enables the operatingmode of the radio digital processor 105. While in operating mode, theradio digital processor 105 is connected to receive a complex modulateddigital signal 110 generated by a baseband processor either directlyfrom the baseband processor or via the source generator 200. Theenvelope and phase components of the complex modulated digital signal110 are input to the radio digital processor 105 for pre-distorting tocompensate for nonlinearities in the PAD 225. Based on the stored errormeasurements, the radio digital processor 105 is able to produce acorrection signal that when added to the complex modulated digitalsignal 110 produces a corrected complex modulated digital signalincluding a corrected envelope signal 115 and a corrected phase signal125 that digitally compensates for the nonlinearities in the PAD 225.

For example, in one embodiment, the radio digital processor 105 can adda DC offset with the opposite sign of the measured LOFT to the envelopecomponent of the complex modulated digital signal to produce a LOFTcorrected signal 205. In addition, the radio digital processor 105 canapply an inverse envelope distortion profile to the envelope componentof the complex modulated digital signal to produce an envelopedistortion corrected signal 210 and can apply an inverse phasedistortion profile to the phase component of the complex modulateddigital signal to produce a corrected phase signal 125. The LOFTcorrected signal 205 can be added to the envelope distortion correctedsignal 210 at an adder 220 to produce the corrected envelope signal 115.

FIGS. 9-11 illustrate exemplary waveforms produced during themeasurement and compensation of power amplifier nonlinearities. FIG. 9is an exemplary waveform illustrating the amplitude of the output of thePAD (e.g., RF signal 148, shown in FIG. 6) during an exemplarymeasurement mode and an exemplary operating mode of the transmitter.Initially, in measurement mode, the LOFT of the PAD is measured bymaintaining a constant envelope test signal at the input of the radiodigital processor. As can be seen in FIG. 9, during the measurement ofthe LOFT, the radio digital processor 105 applies a binary searchalgorithm that successively adjusts the bias of the envelope signal toconverge to a minimum value of DC offset in the output.

Then, during the AM/AM&PM measurement period, the radio digitalprocessor maintains the DC bias of the envelope signal while sweepingthe amplitude through the dynamic range of the transmitter. As can beseen in FIG. 9, the output signal during the AM/AM&PM measurement periodis a stair case curve including sixteen measurement points on theamplitude axis. Thus, at each amplitude measurement point (i.e., eachstep), the radio digital processor compares the feedback signal to thetest signal to measure the AM and PM distortion in the feedback signal.From the measurements taken at each measurement point, the radio digitalprocessor is able to construct the envelope and phase distortionprofiles, such as the profiles shown in FIGS. 7 and 8.

For example, to construct the phase distortion profile shown in FIG. 7,the radio digital processor can employ a quadratic polynomial curve-fitusing the PM distortion measurements taken at each measurement point. Asanother example, to construct the envelope distortion profile shown inFIG. 8, the radio digital processor can employ a linear polynomialcurve-fit using the AM distortion measurements taken at each measurementpoint.

After the AM/AM&PM measurement period, a sawtooth test signal thatsweeps the amplitude range of the transmitter with constant phase isapplied to the radio digital processor in an operating mode tocompensate for the nonlinearities measured in the LOFT and AM/AM&PMmeasurement periods. As can be seen in FIG. 9, there is minimaldistortion in the amplitude of the output test signal after amplitudecompensation.

FIG. 10 is an exemplary waveform of the envelope component of thefeedback signal 120 (i.e., OutMag), shown in FIG. 6. As can be seen inFIG. 10, during the measurement of the LOFT, the DC offset in thefeedback signal varies while the radio digital processor is applying thebinary search algorithm until the DC offset in the feedback signalreaches zero. Then, during the AM/AM&PM measurement period, as can beseen in FIG. 10, the feedback signal contains some AM distortion at eachmeasurement point, which is used to create the AM distortion profileshown in FIG. 8. Finally, during the test signal period, the feedbacksignal also demonstrates minimal amplitude distortion after amplitudecompensation.

FIG. 11 is an exemplary waveform of the phase component of the feedbacksignal 120 (i.e., OutAng), shown in FIG. 6. As can be seen in FIG. 11,phase distortion is present during the AM/AM&PM measurement period, andthe measured distortion can be used to create the PM distortion profileshown in FIG. 7. In addition, as can be seen during the test signalperiod, there is minimal phase distortion present in the feedback signalafter phase compensation.

FIG. 12 is a flow chart illustrating a method 300 for compensating fornonlinearities of a polar transmitter, in accordance with embodiments ofthe present invention. Initially, at steps 310 and 320, during ameasurement mode, a complex digital test signal and a feedback signalproduced from the complex digital test signal are received. At step 330,the complex digital test signal is compared to the feedback signal tomeasure an error signal indicative of a difference between the complexdigital test signal and the feedback signal. For example, when measuringlocal oscillator feed-through (LOFT) of the power amplifier (PA), theerror signal can be a measure of the DC offset or bias that is added tothe envelope path of the transmitter by the PA. As another example, whenmeasuring AM/AM&PM distortion in the PA, the error signal can be ameasure of the variation of the amplitude and phase of the output of thePA as a function of the amplitude of the complex digital test signal. Atstep 340, the error signal measurements are stored for subsequent use inan operating mode of the polar transmitter.

During the operating mode, at step 350, a complex modulated digitalsignal is received, and at step 360, a correction signal is producedbased on the error signal and the complex modulated digital signal.Finally, at step 370, the correction signal is added to the complexmodulated digital signal to produce a corrected complex modulateddigital signal that compensates for nonlinearities (e.g., LOFT, AM/AM&PMdistortion) in the PA of the polar transmitter.

As one of average skill in the art will appreciate, the term“substantially” or “approximately”, as may be used herein, provides anindustry-accepted tolerance to its corresponding term. Such anindustry-accepted tolerance ranges from less than one percent to twentypercent and corresponds to, but is not limited to, component values,integrated circuit process variations, temperature variations, rise andfall times, and/or thermal noise. As one of average skill in the artwill further appreciate, the term “operably coupled”, as may be usedherein, includes direct coupling and indirect coupling via anothercomponent, element, circuit, or module where, for indirect coupling, theintervening component, element, circuit, or module does not modify theinformation of a signal but may adjust its current level, voltage level,and/or power level. As one of average skill in the art will alsoappreciate, inferred coupling (i.e., where one element is coupled toanother element by inference) includes direct and indirect couplingbetween two elements in the same manner as “operably coupled”.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and detailed description. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but, on the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the claims. As may beseen, the described embodiments may be modified in many different wayswithout departing from the scope or teachings of the invention.

1. A radio frequency (RF) polar transmitter, comprising: a digitalprocessor coupled to receive a complex modulated digital signal and afeedback signal and operable to compare the complex modulated digitalsignal to the feedback signal to determine an error signal indicative ofa difference between the complex modulated digital signal and thefeedback signal and further operable to produce a correction signal fromthe error signal and to add the correction signal to the complexmodulated digital signal to produce a corrected complex modulateddigital signal; a Digital-to-Analog Converter (DAC) operably coupled toreceive the corrected complex modulated digital signal and operable toconvert the corrected complex modulated digital signal to a complexmodulated analog signal; a power amplifier operable to produce amodulated RF signal from the complex modulated analog signal; and afeedback loop coupled to receive the modulated RF signal from the poweramplifier and operable to produce the feedback signal from the modulatedRF signal.
 2. The polar transmitter of claim 1, wherein the complexmodulated digital signal includes an amplitude-modulated digital signaland a phase-modulated digital signal.
 3. The polar transmitter of claim1, wherein the Digital-to-Analog converter includes first and secondDigital-to-Analog converters for converting the amplitude-modulateddigital signal and the phase-modulated digital signal, respectively,from analog to digital to produce a phase-modulated analog signal and anamplitude-modulated analog signal, respectively, and further comprising:a translational loop coupled to receive the phase-modulated analogsignal and operable to up-convert the phase-modulated analog signal froman intermediate frequency to a radio frequency to produce aphase-modulated RF signal, wherein the power amplifier is operable toproduce the modulated RF signal from the phase-modulated RF signal andthe amplitude-modulated analog signal.
 4. The polar transmitter of claim2, further comprising: a phase-locked loop coupled to receive thephase-modulated digital signal and operable to produce a phase-modulatedRF signal that tracks the phase of the phase-modulated digital signal,wherein the power amplifier is operable to produce the modulated RFsignal from the phase-modulated RF signal and the amplitude-modulatedanalog signal.
 5. The polar transmitter of claim 2, wherein: thefeedback signal includes a phase-modulated feedback signal and anamplitude-modulated feedback signal; the digital processor is operableto compare the phase-modulated feedback signal to the phase-modulateddigital signal to produce a phase error signal indicative of adifference between the phase-modulated digital signal and thephase-modulated feedback signal and to compare the amplitude-modulatedfeedback signal to the amplitude-modulated digital signal to produce anamplitude error signal indicative of a difference between theamplitude-modulated digital signal and the amplitude-modulated feedbacksignal; the digital processor is operable to produce a phase correctionsignal from the phase error signal and an amplitude correction signalfrom the amplitude error signal; and the digital processor is operableto add the phase correction signal to the phase-modulated digital signalto produce a corrected phase-modulated digital signal and to add theamplitude correction signal to the amplitude-modulated digital signal toproduce a corrected amplitude-modulated digital signal.
 6. The polartransmitter of claim 5, wherein the modulated RF signal includes anamplitude modulated distortion signal produced by the power amplifier;and wherein the digital processor is operable to measure the amplitudemodulated distortion signal as the amplitude error signal and tocompensate for the amplitude modulated distortion signal using aquadratic polynomial curve-fit to produce the amplitude correctionsignal.
 7. The polar transmitter of claim 6, wherein the modulated RFsignal includes a phase modulated distortion signal produced by thepower amplifier; and wherein the digital processor is operable tomeasure the phase modulated distortion signal as the phase error signaland to compensate for the phase modulated distortion signal using alinear polynomial curve-fit to produce the phase correction signal. 8.The polar transmitter of claim 6, wherein the modulated RF signalincludes a local oscillator feed through signal produced by the poweramplifier, and wherein the digital processor is operable to measure thelocal oscillator feed through signal and to compensate for the localoscillator feed through signal by biasing the corrected amplitude signalwith a DC value determined from the local oscillator feed throughsignal.
 9. The polar transmitter of claim 8, wherein the complexmodulated digital signal includes a sequence of test signals in ameasurement mode, and further comprising: a source generator forgenerating the sequence of test signals in the measurement mode and forgenerating a control signal that drives the digital processor during themeasurement mode.
 10. A radio frequency (RF) transceiver, comprising: apolar transmitter including: a digital processor coupled to receive acomplex modulated digital signal and a feedback signal and operable tocompare the complex modulated digital signal to the feedback signal todetermine an error signal indicative of a difference between the complexmodulated digital signal and the feedback signal and further operable toproduce a correction signal from the error signal and to add thecorrection signal to the complex modulated digital signal to produce acorrected complex modulated digital signal; a Digital-to-AnalogConverter (DAC) operably coupled to receive the corrected complexmodulated digital signal and operable to convert the corrected complexmodulated digital signal to a complex modulated analog signal; and apower amplifier operable to produce a modulated RF signal from thecomplex modulated analog signal; and a receiver coupled in a feedbackloop with the transmitter to receive the modulated RF signal from thepower amplifier and operable to produce the feedback signal from themodulated RF signal.
 11. The RF transceiver of claim 10, wherein thecomplex modulated digital signal includes an amplitude-modulated digitalsignal and a phase-modulated digital signal.
 12. The RF transceiver ofclaim 11, wherein the Digital-to-Analog converter includes first andsecond Digital-to-Analog converters for converting theamplitude-modulated digital signal and the phase-modulated digitalsignal, respectively, from analog to digital to produce aphase-modulated analog signal and an amplitude-modulated analog signal,respectively, and further comprising: a translational loop coupled toreceive the phase-modulated analog signal and operable to up-convert thephase-modulated analog signal from an intermediate frequency to a radiofrequency to produce a phase-modulated RF signal, wherein the poweramplifier is operable to produce the modulated RF signal from thephase-modulated RF signal and the amplitude-modulated analog signal. 13.The RF transceiver of claim 11, further comprising: a phase-locked loopcoupled to receive the phase-modulated digital signal and operable toproduce a phase-modulated RF signal that tracks the phase of thephase-modulated digital signal, wherein the power amplifier is operableto produce the modulated RF signal from the phase-modulated RF signaland the amplitude-modulated analog signal.
 14. The RF transceiver ofclaim 11, wherein: the feedback signal includes a phase-modulatedfeedback signal and an amplitude-modulated feedback signal; the digitalprocessor is operable to compare the phase-modulated feedback signal tothe phase-modulated digital signal to produce a phase error signalindicative of a difference between the phase-modulated digital signaland the phase-modulated feedback signal and to compare theamplitude-modulated feedback signal to the amplitude-modulated digitalsignal to produce an amplitude error signal indicative of a differencebetween the amplitude-modulated digital signal and theamplitude-modulated feedback signal; the digital processor is operableto produce a phase correction signal from the phase error signal and anamplitude correction signal from the amplitude error signal; and thedigital processor is operable to add the phase correction signal to thephase-modulated digital signal to produce a corrected phase-modulateddigital signal and to add the amplitude correction signal to theamplitude-modulated digital signal to produce a correctedamplitude-modulated digital signal.
 15. The RF transceiver of claim 14,wherein: the modulated RF signal includes an amplitude modulateddistortion signal produced by the power amplifier; the digital processoris operable to measure the amplitude modulated distortion signal as theamplitude error signal and to compensate for the amplitude modulateddistortion signal using a quadratic polynomial curve-fit to produce theamplitude correction signal; the modulated RF signal includes a phasemodulated distortion signal produced by the power amplifier; and thedigital processor is operable to measure the phase modulated distortionsignal as the phase error signal and to compensate for the phasemodulated distortion signal using a linear polynomial curve-fit toproduce the phase correction signal.
 16. The RF transceiver of claim 15,wherein the modulated RF signal includes a local oscillator feed throughsignal produced by the power amplifier, and wherein the digitalprocessor is operable to measure the local oscillator feed throughsignal and to compensate for the local oscillator feed through signal bybiasing the corrected amplitude signal with a DC value determined fromthe local oscillator feed through signal.
 17. The RF transceiver ofclaim 16, wherein the complex modulated digital signal includes asequence of test signals in a measurement mode, and further comprising:a source generator for generating the sequence of test signals in themeasurement mode and for generating a control signal that drives thedigital processor during the measurement mode.
 18. The RF transceiver ofclaim 10, wherein the receiver further includes: radio circuitry coupledto receive the modulated RF signal from the power amplifier and operableto down-convert the modulated RF signal to an IF analog signal; ananalog-to-digital converter for converting the IF analog signal into anIF digital signal; a filter operable to filter the IF digital signal toproduce a filtered IF digital signal; and a vector de-rotator coupled toreceive the filtered IF digital signal and operable to vector de-rotatethe filtered IF digital signal to produce the feedback signal.
 19. Amethod for compensating for nonlinearities of a polar transmitter,comprising the steps of: receiving a complex digital test signal in ameasurement mode; receiving a feedback signal produced from the complexmodulated digital signal; comparing the complex digital test signal tothe feedback signal to determine an error signal indicative of adifference between the complex digital test signal and the feedbacksignal; storing the error signal; receiving a complex modulated digitalsignal in an operating mode; producing a correction signal based on theerror signal and the complex modulated digital signal; and adding thecorrection signal to the complex modulated digital signal to produce acorrected complex modulated digital signal.
 20. The method of claim 19,wherein said step of receiving the feedback signal further includes thesteps of: converting the complex digital test signal to a complex analogtest signal; amplifying the complex analog test signal to produce a testRF signal; and coupling the test RF signal through a feedback loop toproduce the feedback signal.
 21. The method of claim 20, wherein saidstep of coupling further includes: coupling the test RF signal to areceiver of a transceiver including the polar transmitter to produce thefeedback signal; and coupling the feedback signal to the polartransmitter.
 22. The method of claim 20, further comprising the stepsof: converting the corrected complex modulated digital signal to acomplex modulated analog signal; and amplifying the complex modulatedanalog signal to produce a modulated RF signal.
 23. The method of claim22, wherein the complex digital test signal and the complex modulateddigital signal each include a respective amplitude-modulated digitalsignal and a respective phase-modulated digital signal, and wherein: thefeedback signal includes a phase-modulated feedback signal and anamplitude-modulated feedback signal; said step of comparing furtherincludes: comparing the phase-modulated feedback signal to thephase-modulated digital signal of the complex digital test signal toproduce a phase error signal indicative of a difference between thephase-modulated digital signal and the phase-modulated feedback signal;and comparing the amplitude-modulated feedback signal to theamplitude-modulated digital signal of the complex digital test signal toproduce an amplitude error signal indicative of a difference between theamplitude-modulated digital signal and the amplitude-modulated feedbacksignal; said step of producing the correction signal further includes:producing a phase correction signal based on the phase error signal andthe phase-modulated digital signal of the complex modulated digitalsignal and an amplitude correction signal based on the amplitude errorsignal and the amplitude-modulated digital signal of the complexmodulated digital signal; and said step of adding further includes:adding the phase correction signal to the phase-modulated digital signalof the complex modulated digital signal to produce a correctedphase-modulated digital signal; and adding the amplitude correctionsignal to the amplitude-modulated digital signal of the complexmodulated digital signal to produce a corrected amplitude-modulateddigital signal.
 24. The method of claim 23, wherein the test RF signalincludes an amplitude modulated distortion signal produced by said stepof amplifying the complex analog test signal; and wherein said step ofproducing the correction signal further includes: compensating for theamplitude modulated distortion signal using a quadratic polynomialcurve-fit of the amplitude error signal to produce the amplitudecorrection signal.
 25. The method of claim 24, wherein the test RFsignal includes a phase modulated distortion signal produced by saidstep of amplifying the complex analog test signal; and wherein said stepof producing the correction signal further includes: compensating forthe phase modulated distortion signal using a linear polynomialcurve-fit of the phase error signal to produce the phase correctionsignal.
 26. The method of claim 20, wherein the test RF signal includesa local oscillator feed through signal produced by said step ofamplifying the complex analog test signal, and wherein said step ofproducing the correction signal further includes: compensating for thelocal oscillator feed through signal by biasing the corrected amplitudesignal with a DC value determined from the error signal.
 27. The methodof claim 19, wherein the complex digital test signal includes a sequenceof test signals, and further comprising the steps of: generating acontrol signal to initiate a measurement mode; and generating thesequence of test signals in the measurement mode.